An electric motor energized by a power source such as a battery loaded on the vehicle is normally used as drive source of a wiper device of a vehicle such as an automobile. Such an electric motor is equipped with a speed reduction mechanism for reducing the number of revolutions of the output shaft thereof to a predetermined number of revolutions, the motor is unitized as an electric motor with the speed reduction mechanism. One or two such motor units are used for a wiper device and a wiper arm is driven by the drive source of the motor unit or units to swing between an upper reversal position and a lower reversal position where it turns around. When a single motor unit is used, the wiper arm at the driver's seat and the one at the passenger seat are combined by a link and driven synchronously. When two motor units are used, the wiper arm at the driver's seat and the one at the passenger seat are connected to the respective motor units and driven synchronously, while detecting the revolutions of the armature shaft and those of the output shaft.
Meanwhile, the space for installing a wiper system has been reduced year by year as a result of the increase in the dimensions of the engine and also in the master power of the brake. As a result, there has been marketed a system that can drive a wiper within a small space by reversing the motion of the motor by less than 180° in order to reduce the operating area of the link to less than a half of the ordinary area. Since such a motor reversing system can reverse the moving direction of the wiper at any desired position within the wiping angle of the wiper, it is possible to define the lower reversal position and set a wiper storing space below the defined position. Many high quality cars currently adopt the system and are provided with such a wiper storing feature.
When reversing the motion of the motor of a wiper system, it is necessary to detect the wiper arm position in order to reverse the motion of the motor at a desired position of the wiper arm. The wiper arm position is detected by adding/subtracting the number of pulses generated by a pulse generator whose operation is interlocked with the rotary motion of the motor. A multi-polar magnet is fitted to the rotary shaft of the motor and a sensor typically comprising Hall ICs is arranged vis-à-vis the magnet in order to detect the polar change due to the rotation of the rotary shaft and to output a pulse signal. The pulse count is reset at a point (origin position) that operates as reference position for the rotary position of the output shaft of the motor unit in order to prevent discrepancies of pulses. A magnet is also fitted to the output shaft and a corresponding sensor is arranged in such a way that the sensor outputs a reference signal when a magnetic pole passes by a predetermined position.
The rotary angle of the motor from the reference position is computed by adding/subtracting the number of pulses after the reset so that it is possible to detect the current wiper arm position by taking the reduction ratio and the link ratio into consideration. It is also possible to detect the rotary speed of the moving wiper arm from the cycle period of pulses generated due to the rotary motion of the motor. The motor control system is provided with a reversing circuit which may typically be an H-bridge circuit that comprises FETs and a control means such as a CPU for controlling the rotary speed and the rotary angle of the motor so that the operation of driving the motor is controlled according to the wiper arm position and the rotary speed of the wiper arm.
In such a known wiper system that is adapted to be driven to operate by a reversible motor, if an abnormal condition arises because of a power shut down or some reason on the way of a wiping operation, the pulse count that indicates the wiper arm position can be lost to make it impossible to accurately recognize the wiper arm position when the wiper system is restarted. Then, the wiper blade can overrun to collide with the corresponding pillar at an end of the windshield and/or a component of the reduction mechanism or the link mechanism can hit the mechanical stopper arranged in the motor unit.
When it snows, snow flakes can be accumulated on the wiper blade so that it is sometimes very difficult to start driving the blade and move it from the stored position. Then, the car driver may often have to drive the blade to reciprocate between the stored position and the lower reversal position manually or automatically in order to activate the wiper device. However, when the motor is operated for forward rotation and reverse rotation repeatedly, the pulse count that indicates the wiper arm position can become to show discrepancies. Then, as discrepancies are accumulated, the operation of the blade can become unstable.
In the above-described wiper system, a sensor for detecting the rotary position of the output shaft is arranged not only at the origin position but also at the upper and lower reversal positions and at the stored position in order to detect the wiper arm position at a number of locations for the purpose of stabilizing the operation of the blade. Thus, an arrangement is made to quickly recognize the wiper arm position and prevent it from overrunning or otherwise operating unstably if the system is restarted without recognizing the position at which the wiper arm is stopped or if the pulse count gives rise to discrepancies and they are accumulated. However, with such an arrangement, since it is necessary to install at least four expensive sensors for a single motor, there is a problem that a unit price increases and it causes a cost rise.
An object of the present invention is to provide a wiper device control method, and the like that can reliably detect the wiper arm position with a small number of sensors.